Aircraft density and midair collision

The frequency of near midair collisions under visual flight rules (VFR) as a function of aircraft density is derived from a model which correlates closely with recent experience. Examples are given which indicate the degree of air traffic control (ATC) needed for safe separation at various levels of aircraft density for both unordered and ordered flight. It is argued that loose forms of ATC such as intermittent positive control can be used safely to prevent collisions while preserving most of the freedom of VFR flight at densities as high as those predicted for peak hours in the Los Angeles basin in 1995.